Many industrial methods require particles to be dried before their subsequent use, whether before the packaging of granular agri-foodstuff products or of industrial products, or before the combustion of ground waste used as fuels. It is of course possible to perform the drying of the particles in batches by depositing the particles on decks or in a rotary drum, preferably perforated in order to allow a hot gas to pass through and allow water and steam to escape. In some cases, a fluidized bed is formed by the particles in suspension under the action of the flow of hot gas. However, most of the industrial applications require flow rates that a batch drying method cannot achieve. For this reason, the same principle of depositing the particles to be dried on a perforated support and of exposing them to a flow of hot gas has been applied to apparatus used for continuous drying, with a continuous source of the particles to be dried upstream of the dryer itself and a continuous discharging of the dried particles downstream thereof.
In particular, a belt dryer is schematically illustrated in FIG. 1(a) and comprises a continuous perforated flexible belt stretched between two motor-driven rollers forming a loop. Air or other hot gas is blown under the upper cloth on which the particles to be dried are continuously deposited. The length of a belt dryer depends on the type of particles to be dried and their water content. Typically, if a surface area of 120 m2 is necessary to dry the particles at the desired speeds, the belt will have to have a surface area at least two times greater, of the order of 250 m2, because the particles are dried only on the top part of the loop linking the two rollers. For a width of 2.5 m, a belt 200 m long would therefore be required to link two rollers approximately 80 m apart. A belt of such dimensions is very expensive, and difficult to mount/remove on the apparatus. A belt dryer is therefore generally reserved for the drying of a single type of particles, because it would be uneconomical to change the belt to optimize the type of perforation to a new type of particles. Should the belt be damaged, the entire unit has to be stopped for a long time, the time it takes to change or repair the belt. To support a belt over such a length, many support rollers mounted on bearings are necessary, which increases the cost and also the risks of failure of such an apparatus. A belt dryer is therefore very costly and ineffective in terms of dimension, since the particles are dried only over less than half of the length of the belt.
There are also perforated deck dryers as represented schematically in FIG. 1(b), which resemble belt dryers, except that the belt is replaced by perforated decks coupled to one another forming a sort of caterpillar track. The difference with a belt dryer is that the decks are articulated so as to present the same face whether they are the top or bottom belt of the loop. This makes it possible to reduce the length of the dryer practically by half, since the particles are subjected twice to a flow of hot gas: the first time in their passage over the top part of the loop and the second time in their passage in the reverse direction over the bottom part. Although advantageous from this point of view relative to a belt dryer, it is clear that the mechanics necessary for the movements of the decks are delicate and therefore costly and fragile, especially when exposed to fine particles that can seize up the rolling bearings. Furthermore, the openings created between two adjacent decks and, above all, the spaces opening up in the deck transfer mechanism in each transfer of a deck from the top portion to the bottom portion of the caterpillar track create so many preferential passages of lesser resistance for the flow of hot gas, which result in a significant drop in the efficiency of this type of dryer.
EP197171 describes a dryer represented schematically in FIG. 1(c) (without the powder distribution and recovery means to simplify the figure) and comprising a number of superposed, circular, perforated decks (1a, 1b) mounted to rotate on a hollow central axis. Each deck is enclosed in an individual cylindrical chamber provided with a roof [18] and a floor which separate it from the other decks. Means (4a) for transferring the powder to be dried are provided between each adjacent deck (see gray arrow (4a)). Each chamber is provided, on the one hand, with a first hot air intake opening, fluidically connected with the cavity of the hollow central axis, said first opening being positioned above the deck located in the corresponding chamber and, on the other hand, a second exhaust opening on the peripheral wall of the chamber connected with the outside (or a hot air exhaust system), said second opening being located under the corresponding deck. Hot air is blown into the cavity of the hollow axis according to the black arrows in FIG. 1(c) and is distributed in parallel into each chamber through the first hot air intake opening. The hot air has to pass through the circular perforated deck before being discharged through the second opening located on the peripheral wall of each chamber. In reality, such a system is similar in principle to a belt dryer (see FIG. 1(a)), whose linear motion has been replaced by a circular motion distributed over a plurality of levels with means for transferring powder from one deck to another. Such a rotary system does indeed have a considerable advantage of space-saving on the ground compared to a linear belt dryer, but such a system lacks efficiency. In practice, while the hot air having passed through the first decks loaded with very wet particles emerges relatively saturated with moisture, the hot air passing through the final decks loaded with particles already partially dried on the preceding decks emerges with very little moisture content, which represents a considerable waste of energy.
There therefore remains a need for an industrial dryer for drying particles continuously which is effective, easy to maintain, occupying less space on the ground and less expensive. The present invention proposes such an industrial dryer.